Cathode for thermionic tubes



April 5 1932. H. w PARKER CATHODE FOR THERMIONIC TUBES Filed July 25,1930 Fis- 4:

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.RNEY' Patented Apr. 1932 UNETED STAT HENRY W. PARKER, OE TORONTO,ONTARIO, CANADA, ASSIGNOR TO ROGERS RADIO C TUBES, LIlVIITED, OFTORONTO, ONTARIO, CANADA, A CORPORATION OF ONTARIO GATHODE FORTHEBMIONIO TUBES Application filed m 25, 1930/ Serial No. 470,625.

My invention relates to cathodes for electron discharge devices such asthermionic tubes and pertains more specifically to that type of cathodetermed equi-potential and 5 employing separate heating and emittingmembers.

An object of my invention consists in providing a cathode of theindirectly heated type in which the magnetic field in the proximity ofthe cathode produced by the alternating heating current is reduced tozero.

Another object contemplates providing a cathode through the use of whichan alternating current heated thermionic tube may be constructed whichexhibits no disturbing hum due to modulation of the space current of thetube by magnetic strays from the heating current.

, A still further object consists in providing a cathode which willpermit of the construc tion of thermionic tubes having a higher qualityof output.

I accomplish all these and other desirable I features which will behereinafter pointed out and discussed by reducing the magnetic field inthe proximity of the heating element to exactly zero by employing aheater in the form of a heating wire maintained co-axially within aconducting cylinder which likewise functions as a portion of the returncircuit for the current traversing the heater wire.

Many attempts have hitherto been made to reduce the hum in thermionictubes employing indirectly heated cathodes and up to the present timethe most successful type of hum eliminating cathode utilizes a heater.consisting of a wire shaped to form a loop. This so-called hairpin typeof heater eliminates a certain portion of the stray magnetic fieldproduced by the alternating heating current, but does not reduce suchstray to zero.

With my improved form of cathode, I achieve complete and absoluteelimination of magnetic strays from the alternating heating current andthus through the use of such cathode thermionic tubes exhibiting ahum-free output of higher quality may be constructed.

In the drawings accompanying and forming a part of this specification,in which like numerals designate corresponding parts throughout Fig. lis an enlarged sectional elevation of one embodiment of my improvedcathode. K Fig. 2 is an enlarged sectional elevation of an alternativeformof my cathode.

Fig. 3 is a cross-section taken on the line 33 of Fig. 2.

Fig. 4 is an enlarged sectional elevation of bring out the structuralfeatures of my in- "1.

vention:

Fig. 1 illustrates an embodiment of my idea in which the heater wire 1,composed. of tungsten or other material which is rendered incandescentby the passage of electric current therethrough, is co-axially disposedwithin preferably a nickel cylinder or tube 3, the upper extremity ofwhich is tapered to a point 2 as shown. The heater wire 1 is welded orotherwise conductively connected to the tapered point 2 and a terminal 4is provided for the tube. The tube 3 functions as a portion of thereturn circuit for the heating current and as it completely encirclesthe heater wire 1, obviously there will exist outside of the tube nomag,- netic stray field resulting from the passage of the alternatingheating current there through. A cylinder 6 of refractory mate' rialsuch as porcelain maintains the heater wire 1 in co-axial alignment withthe tube 3. A metallic cylinder 8, provided prefer; ably with a closedtop 14:, is co-axially maintained with respect to the heater wire 1 andtube 3 by means of the cylinder of the refractory material 7. Cylinder 8is provided with a coating of electron emitting substance or substancessuch as strontium or barium oxide or a mixture thereof and constitutesthe element from which electrons are emitted can! through the agency ofthe heat developed by the ineandescence of the heater wire 1.

In Figs. 2 and 3 I have shown another embodiment of my idea in which therefrac tory cylinder 6 is omitted and the heater wire is maintained inco-axial relationship with the return circuit tube 3 by tension or bythe employment of a heater wire of proper dimensions to exhibit thenecessary rigidity to accomplish the purpose. The cylinder 8 carriesthereon electron emitting substances 9 and is separated from the tube 3by the cylinder 7 of refractory material. A connection 5 to the cylinder8 is provided for extension through the press of the ther mionic tubethat electrical connectionmay be made with the active portion of thecathode.

Referring to Fig. 4, I have illustrated a still further embodiment of myidea wherein the electron emitting substance or substances are carriedby a perforated cylinder 18. The use of t e perforated cylinder is notnecessary but constitutes an embodiment whereby heat transfer from theheater wire 1 to the electron emitting substances 9 is facilitated.

The length of the present day heater is governed by the alternatingcurrent voltages used and the physical constants of the tungsten heaterwire and it is not possible to use the standard heater wire length withmy coaxial cylinder construction. In order to obtain the same length ofheater wire as is now common in standard thermionic tubes, it isnecessary in my construction to use two heaters connected in series asshown in Fig. 5. Heater wire 1 is connected to return co-axial cylinder3, the lower extremity of which is connected through conductor 10 withthe lower extremity of cylinder 12 to the top of which is attachedheater wire 11. The twin heaters thus formed are surrounded byrefractory material 17 which carries a metallic sleeve 28 hearingelectron emitting substances 19 thereon and a connection wire 15attached thereto.

With this type of construction, my improved cathode may be adapted topresent day Voltages in keeping with standard heater wire lengths. Inother words, the embodi- Vment of my invention just described may beutilized in standard thermionic tubes without requiring any change ofvoltage in the heater supply to permit the construction of a thermionctube which may be used in standardized circuits and with standardizedequip ment.

It will be noted that my improved cathode reduces magnetic strays fromthe alternating heating current to zero as no magnetic stray can existoutside of the enclosing metallic tube 3 which functions as the returncircuit for all of the heating current traversing the heating wire 1 andthat thus I have produced a cathode by which thermionic tubes may heconstructed which exhibit no hum due to magnetic strays from the heatingcurrent.

As noted hereinbefore, I have purposely exaggerated the diameters of thereturn tubes and the electron emissive cylinders in order that theconstruction of my cathode may be more clearly shown. Obviously, thediameters of the elements of my cathode must be so proportioned that theproper activation temperature may be obtained, butsuch requirement willbe readily understood by anyone skilled in the art.

Having thus completely described my invention by the illustration anddescription of several embodiments thereof, what Iclaim as new anddesire to secure by Letters Patent of the United States is as follows: 7

1. A cathode comprising, a pair of metal lic cylinders conductivelyconnected to each other, a heater wire co-axially disposed within eachof said cylinders and conductively connected thereto and an electronemitting element disposed adjacent to said cylinders.

2. A cathode comprising, a plurality of cylindrical conductorsconductively connected to each other at one of the extremities thereof,a heating element co-axially disposed within each of said conductors andconductively connected thereto and an electron emitting elementenclosing said cylindrical conductors. r

8. A. cathode comprising, a pair of hollow cylindrical conductors oneextremity of each of which is closed and the other extremities of whichare conductively connected to each other, a heating element C o-axiallydisposed within each of said conductors and conductively connected tothe closed extremities thereof and an electron emitting element dis- Iity and conductively connected to each other i at the other extremitiesthereof,'a heating element co-axially disposed Within each of saidconductors and conductively connected respectively to the closedextremities thereof and a metallic member bearing electron emittingsubstances surrounding said cylindrical conductors.

HENRY w. PARKER,

